In an internal combustion engine, such as a diesel engine or a gasoline engine, in general, an air-fuel ratio (the ratio between air (oxygen) and fuel fed to combustion chambers) is adjusted in order to reduce fuel consumption or purify exhaust gas. In order to control the air-fuel ratio, there is used a measurement signal from a gas sensor which measures the ratio of oxygen contained in intake gas or the ratio of oxygen contained in exhaust gas.
The above-mentioned gas sensor is mainly composed of a sensor element for measuring the concentration of oxygen; a protection cover which covers the circumference of the sensor element; a metallic shell which holds the sensor element; and a base portion which includes a plurality of terminals for leading a measurement signal from the sensor element to an external circuit. Conventionally, the base portion is fabricated by assembling a plurality of thin metal sheets. Therefore, the conventional gas sensor has a problem in that the assembly is troublesome and time consuming.
In order to solve such a problem, there has been proposed a technique of insert-molding a base portion formed of resin integrally or unitarily with a metallic shell, etc. (see, for example, Patent Document 1). By means of forming the base portion from resin to be united with the metallic shell, etc., the work of assembling a plurality of thin metal sheets can be simplified.